Waste fluids that contain cyanide ions are typically produced by cyanide refining, and other industrial waste fluids sometimes contain cyanide. The waste fluid referred to here is typically waste water or waste slurry. Moreover, the metallic cyano complex ions that are difficult to decompose referred to here are typically iron cyano complex ions and nickel cyanide ions.
The cyanide refining method is one method for recovering gold from gold ore. In this method, for example, the gold ore is crushed, then the gold is in contact with the solution that contains cyanide ions and leached out as gold cyano complex ion, and then finally the gold is recovered from the solution after leaching. In this method the cyanide ions are used repeatedly. This cyanide refining method is considered to be an excellent method from the aspect that it is possible to recover even small amounts of gold.
In the cyanide refining method which uses cyanide ions, there is always the danger that the cyanide ions will be included in the water that adheres to the leaching residue that occurs, or that the cyanide ions will be included in the surplus water, site water or the like, and so processing the waste fluid which includes cyanide ions that come from the cyanide refining method has become a large problem.
Up until now, to overcome this problem, it was normal practice that the waste fluid was made harmless by using various methods that are mentioned later, and then the waste fluid was diluted with large amounts of water before discharging it outside of the plant. However, in order to conform to the recent movements which emphasize the importance of environment such as reviewing environmental standards, these methods are not sufficient, and in plants where it is only possible to use limited plant water, this has become a major problem, and it has become difficult to adopt the cyanide refining method.
The natural decomposition method, acid evaporation recovery method, hydrogen peroxide method, alkali chlorine method and SO.sub.2.AIR method have been used or tried in the past to decompose the cyanide ions in the waste fluid.
In the case of the natural decomposition method, a dam or the like is constructed next to the refinery, and the leaching residue or waste fluid which contains cyanide ions is temporarily stored in this dam and left for several months and decomposed through natural purification and the water itself is allowed to evaporate. In this natural decomposition method, it is known that since only natural purification capabilities are used, a very large dam is required, and that for example, in the winter, the purification speed is greatly reduced when the water in the dam freezes. Furthermore, from the aspect of protecting the environment, using this method as the sole processing method is seen as a problem.
With the acid evaporation recovery method, acid is added to the waste fluid to lower the pH, and then the cyanide ions are transformed to hydrogen-cyanide and evaporated, and absorbed by an alkali or the like for recovery. In this method, since an acid is used, it is possible to remove not only the free cyanide ion in the waste fluid, but also all of the metallic cyano complex ions which includes the iron cyano complex ion. However, from facilities and economic or efficiency point of view, it is not practical to use this method for processing the leaching residue that occurs during the cyanide refining process, and so it rarely used.
With the alkali chlorine method, hydrogen peroxide method and SO.sub.2.AIR method, the cyanide ions in the waste fluid are oxidized to be decomposed, and in these methods chlorine, hypochlorous acid, hydrogen peroxide or air are used as a direct oxidizing agent. In particular, the alkali chlorine method and SO.sub.2.AIR method have a good record.
Of these three methods, from the aspect of safety of the by products and the processing cost, the SO.sub.2.AIR method is the most favorable, and in newly developed god mines, the SO.sub.2.AIR method is often used.
What can be said safely for a common characteristic among the three methods mentioned above is that the main decomposable component is the free cyanide ion, and it is not always possible to decompose all of the metallic cyano complex ions. For example, if the iron grade in the gold ore is high, the free cyanide ion bonds to the iron to form iron cyanide complex ions, so the concentration of iron in the solution after leaching may be high. The iron cyano complex ion is the most difficult of all the cyanide compounds to decompose, and with the hydrogen peroxide method it is not possible to decompose the iron cyano complex ion, and decomposition of nickel cyano complex ion is also incomplete.
Even in the alkali chlorine method, it is difficult to decompose the iron cyanide complex ion. In these examples, it is not possible to decompose the iron cyano complex ions because there exists a very strong bonding force between the iron atoms and cyanide ions. Thus, decomposition will not be possible if oxidation is not strong enough to break such bonding force.
On the other hand, the SO.sub.2.AIR method does not decompose the iron cyano complex ion through oxidation with air, but the iron cyano complex ion is caused to react with copper ions to form copper ferrocyanide, and this is precipitated out and removed. However, if there is not enough copper ions in the waste fluid, it is necessary to add copper ions from the outside. Also, even if precipitation is possible by adding copper ions, the cyanide is simply separated through precipitation, and permanent removal by decomposition of the cyanide ions is not possible.
In this way, it can be said that in the SO.sub.2.AIR method the iron cyano complex ion is separated and removed, but in actuality, the cyanide ions continue to exist after separation, and is partially decomposed by ultra violet rays at the location where the precipitate that occurred during processing is controlled, and then flows out as metallic cyano complex ions.
Of these, decomposing cyanide ions from the waste of the cyanide refining has been indicated as a possibility in North America since the middle of the 1970s. However, in research reports up until now, it was reported that there is a possibility of ozone decomposition of free cyanide ion and relatively easily decomposed metallic cyano complex ions, but that ozone decomposition of iron cyano complex ion, which is difficult to decompose, is not possible.
In regard to ozone, before 1980 it was only possible to obtain gas containing ozone (called ozone gas below) with an ozone concentration of 20 g/m.sup.3 or less, however in the 1980s, ozone gas with an ozone concentration of 40 g/m.sup.3 was actually being used by research organizations.
However, even when using this kind of ozone gas with a high ozone concentration, the decomposition reaction with iron cyano complex ion is reported to have only proceeded to a concentration of iron cyano complex ion that was half the initial concentration when several hours were used as the reaction time, and even today this method has yet to reach industrial standards.
In the materials from the "Ozone World Conference in Kyoto" that was held in 1997, it was reported that iron cyano complex ions were decomposed in the neutral pH region. This was a report of the results of the test performed by Ruhle et al. of decomposing iron cyano complex ions under the conditions of; pH7, ozone filling speed of 9 mg/liter-min at 24 liters/hour of ozone gas (according to the inventors calculation, the ozone concentration of the ozone gas was estimated to be 22.5 g/m.sup.3), however, after a reaction time of 180 minutes, approximately 60% of the initial concentration was finally decomposed, leaving approximately 40% that still not decomposed.
In the report examples mentioned above, the decomposition speeds at each pH level of the iron cyano complex ion are not compared, and nothing has been mentioned about pH and the effect that pH has on the decomposition of iron cyano complex ion. Moreover, in none of the reports, including the report by Ruhle et al., was there any suggestion that there was a relationship between the decomposition of iron cyano complex ion and pH.
Recently, from the view point of environmental protection, there is a desire to find a higher-level decomposition processing method for decomposing and processing the cyanide ions that exist in the waste fluid from cyanide refining in newly developed methods of gold mining. As a result, there are expectations to find a decomposition method that is more efficient and effective than the SO.sub.2.AIR method. Not only is there a search to find a method for decomposing iron cyano complex ion, but there is a need to seriously consider measures for protecting the environment in overall mining development, including, for example, the problem of acidification of already processed waste fluid that is generated as a result of oxidizing the surplus SO.sub.2 in the SO.sub.2.AIR method used up until now (the waste water comes from cyanide processing is changed to a sulfate acid, and cannot be disposed of as is), or the problem of handling the thiocyanic ions that generate cyanide and occur due to decomposition that has only proceeded 10% by the SO.sub.2.AIR method, or furthermore the problem of safely transporting the raw SO.sub.2 material used as the processing agent.
The method of processing cyanide ions including iron cyano complex ion in cyanide refining waste with ozone has been avoided because of the initial investment cost for ozone generation equipment and the high cost of generating ozone. However, as a result of recent technological developments, the required amount of electric power required to generate one unit of ozone has greatly decreased, so cost wise it is becoming more inexpensive. However, actually, there has not been enough investigative research performed to obtain basic data regarding the decomposition of iron cyano complex ion, and establishment of a technologically practical and detailed method for processing the aforementioned waste with ozone is still a long way off.
When processing and decomposing cyanide ions with ozone with respect to the waste water that occurs during cyanide refining, it is necessary to reconsider the processing method so that it includes decomposing not only free cyanide ion but also the metallic cyano complex ions that are difficult to decompose. To do this, it can be imagined that the process will be divided into two or more steps when separate processing methods are used to process the free cyanide ion and the difficult to decompose metallic cyano complex ions. If that is the case, then important points to be considered are first, to have sufficient decomposition capability, and second, how to reduce the overall processing cost while at the same time keeping the reaction speed high.